1. Field of the Invention
The present invention relates generally to an image forming apparatus, such as a printer, facsimile and so forth employing an electrophotographic process. More particularly, the invention relates to an image forming apparatus which can maintain supply of toner for a latent image carrier body in stable state for a long period.
2. Description of the Related Art
The conventional image forming apparatus has a photo conductor drum (latent image carrier body), a developer and a power source. The developer includes a developing roller supplying a toner to the photo conductor drum, a toner supply roller supplying the toner to the developing roller and a thin film forming member forming a thin film layer of the toner supplied to the developing roller for restricting a toner deposition amount and a charge amount for the developing roller. To the toner supply roller and the thin film forming member, a bias voltage and current from the power source is applied. On the other hand, to a rotary shaft of the developing roller, a constant voltage is applied.
Upon developing a latent image on the photo conductor drum with the toner, the toner to be supplied onto the developing roller by rotation of the toner supply roller and the developing roller with sliding contact therebetween, is formed into a uniform thin film by the thin film forming member. At this time, the toner is charged by frictionizing between the toner supply roller and the developing roller and by frictionizing the thin film forming member and the developing roller, and then is supplied to the photo conductive drum.
FIG. 8 is a graph showing a charge bias characteristics in the conventional image forming apparatus. A hatched area in the graph shown in FIG. 8 shows a region A where printing (photographic printing) is good in quality. A graph shown by a wavy line is the first prior art in a case where a constant voltage control is performed at -600V. In this prior art, by supplying a constant voltage at -600V to the toner supply roller and the thin film forming member, a fluctuation of a charge amount of the toner on the developing roller can be reduce, and in conjunction therewith, lowering of the charge amount of the toner on the developing roller associated with printing can be restricted. In general, as a value of the constant voltage, the same value as a constant voltage Vb applied to the developing roller or a greater value than the constant voltage Vb is used.
On the other hand, a graph shown by one-dotted line represents the second prior art and illustrates a characteristics of an image forming apparatus of the type, in which a constant current is applied to the toner supply roller and the thin film forming member, as disclosed in Japanese Unexamined Patent Publication No. Heisei 9-106172. In the second prior art, by supplying a constant current of 3 .mu.A! to the toner supply roller and the thin film forming member, the similar effect as that of the first prior art can be obtained.
In the first and second prior arts, an impedance characteristics of a developer in the vicinity of lift time as use limit is risen to be higher than that in the case of the brand new condition immediately after starting of use.
FIG. 9 is a graph showing a correlation between a potential difference and fogging of toner. In FIG. 9, smaller value of fogging of toner represents lesser fogged portion of toner (contamination) to achieve better image. Smaller potential difference .vertline.Vsr-Vb.vertline. between the voltage Vsr applied to the toner supply roller and the thin film forming member and an applied voltage Vb for the developing roller, it can be appreciated from the graph that a charge amount of toner is lowered to frequently cause white fogging depositing toner in the white blank portion, or so forth to degrade printing quality. FIG. 10 is a graph showing a relative relationship between the current value and the fogging of toner. From this graph, when a current Isr applied to the toner supply roller and the thin film forming member becomes smaller, the charge amount of toner is lowered to frequently cause white fogging depositing toner in the white blank portion, similarly to the case of the potential difference.
As set forth above, when the voltage Vsr or the current Isr is increased, an applied voltage Vb for the developing roller is influenced so that the applied voltage Vb, voltage Vsr or the current Isr can not be constant value to constantly cause disturbance of waveform. By this, it becomes difficult to charge the toner to uniformly deposit the toner on the developing roller.
FIG. 11 is a graph showing a correlation between a product of the voltage Vsr and the current Isr and an Isr amplitude value. In FIG. 11, a disturbance of waveform is expressed by an amplitude value .DELTA. of the Isr current waveform and the product of the voltage Vsr and the current Isr is taken as an absolute value Wsr mW!. From the graph, it can be appreciated that the amplitude value .DELTA. tends to increase associating with increasing of the absolute value Wsr.
For example, when the developer has the developer impedance characteristics in the firs prior art of FIG. 8, in order to obtain good printing condition by applying a voltage of -400V!, it becomes necessary to apply a current Isr of greater than or equal to 3.0 .mu.A! to the toner supply roller and the thin film forming member.
Here, discussion will be given, as one example, for the case where an absolute value Wsr as a product of the voltage Vsr and the current Isr to be applied to the toner supply roller and the thin film forming member is less than or equal to 2.5 mW!. For example, in the first prior art illustrated in FIG. 8, a bias to be applied to the toner supply roller and the thin film forming member is controlled at a constant voltage of -600V!. Therefore, at the initial stage of use (brand-new state), the current Isr of about 4.0 .mu.A! becomes necessary, and at the end stage of use (close to end of life), the current Isr of about 2.3 .mu.A! becomes necessary. In this case, while the absolute value Wsr mW! does not exceed 2.5 mW! at both of the initial stage and the end stage, the current Isr becomes lower than 3.0 .mu.A! at the end stage. Namely, since the current Isr to be supplied to the toner supply roller and the thin film forming member becomes smaller according to increasing of number of times of printing, it is quite difficult to restrict lowering of the charge amount of the toner on the developing roller. As a result, dot sharpness can be degraded to degrade printing quality to lower resolution of half-tone meshing or to cause thickening of character. On the other hand, it is also cause degradation of toner fogging phenomenon depositing toner to the portion to be white blank portion where the toner should not be deposited.
On the other hand, in the case where the developer has the developer impedance characteristics in the second prior art illustrated in FIG. 8, in order to obtain a good printing state by supplying the current Isr of 3.0 .mu.A!, it becomes necessary to apply the voltage Vsr of about -550V! at the initial state and the voltage Vsr of about -650V! at the end state, commonly to the toner supply roller and the thin film forming member. In this case, the voltage Vsr to be applied to the toner supply roller and the thin film forming member becomes greater according to increasing of number of times of printing to possibly cause defects similarly to that set forth above.